INTRODUCTION TO THE AUTONOMIC NERVOUS SYSTEM

LECTURE No. 7

INTRODUCTION TO AUTONOMICSGeneral Characteristics of ANSINVOLUNTARYEFFERENT (MOTOR) SYSTEM; ( Plus SENSORY hitchhikers)VISCERAL - innervates 3 target tissues:smooth muscle (GI tract, blood vessels, etc)cardiac muscle (and pacemaker / conduction tissue)glands (sweat, mucous, lacrimal, etc.)

TWO MAJOR COMPONENTS:a) sympathetic (SNS = thoracolumbar)b) parasympathetic (PNS = craniosacral)minor third component that is gut-related:c) entericTWO NEURON SYSYEM from CNS to target structures:a) preganglionic neuron (cell body in CNS)b) postganglionic neuron (cell body in autonomic ganglion)Somatic vs. AutonomicBody Wall & Limbs Visceral (glands, smooth and cardiac muscle) Motor SensoryParasympathetic SympatheticRest / Digest Fight / FlightSkeletal m.PainPressureProprio-ceptionTempTouchVoluntaryInvoluntaryexternal environmentinternal environmentGlandsCardiac m.Smooth m.Body CavitiesEverywhereMotor MotorGlandsCardiac m.Smooth m.SensoryPainPhysiological (reflex)

Thoraco-lumbar(T1-L2)SNSPSNSCranio-sacral(S2-4)(CN 3,7,9,10)Sympathetic chain and paravertebral (chain) ganglia - cervical to sacralPre-aortic sympathetic gangliaOutflow of autonomics from CNSCranio - from the brain: preganglionic cell bodies in brainstem nuclei associated with CN III, VII, IX, X (Vagus)Vagus is the only cranial parasympathetic cranial nerve functioning in the cervical, thoracic and lumbar regions, serving parasympathetics to visceral structures in the neck, thorax and abdomen

Sacral - sacral or pelvic region: preganglionic cell bodies in spinal cord segments S2,3,4 - functioning in the pelvic region (called pelvic splanchnic nerves)

There is NO PARASYMPATHETIC INNERVATION TO THE BODY WALL OR LIMBS (therefore parasympathetic fibers should only be found on nerves leaving the CNS - CN III, VII, IX and X and S2-4, and then distributed in plexuses around organs in the body cavities)Parasympathetic - Craniosacral5Sympathetic Thoraco-lumbar1) Preganglionic cell bodies are located in lateral horn of spinal cord segments T1 L22) All preganglionic axons enter the sympathetic chain through white rami communicans3) From the above outflow into the sympathetic chain, sympathetics are supplied to the entire body, including the body wall/limbs as well as organs in body cavities (therefore there must be routes from the chain that lead to body cavities as well as the body wall)Visceral afferents - sensory fibers that accompany both sympathetic and parasympathetic fibers (these visceral afferents are hitch-hiking with sympathetics and parasympathetics and are not technically part of the SNS or PSNS)Autonomic Ganglia - autonomic ganglia contain postganglionic sympathetic or parasympathetic nerve cell bodies Sympathetic Ganglia1) Sympathetic chain ganglia (paravertebral ganglia) chain ganglia are found at all levels of the spinal cord / spinal nerves2) Preaortic sympathetic ganglia (prevertebral ganglia) these are only located in the abdomen, associated with major branches of the abdominal aorta

Parasympathetic Ganglia discrete parasympathetic ganglia are only found in the head; FOR THE REST OF THE BODY, postganglionic parasympathetic cell bodies are found scattered near or in the walls of target organsAutonomics Schematic CNSPNSSympatheticParasympatheticCell bodies in CNS nucleiCell bodies in PNS gangliaTarget:Smooth m.Cardiac m.GlandsTarget:Smooth m.Cardiac m.GlandsShortShortLongLong

8 Cervical Segments12 Thoracic Segments5 Lumbar Segments5 Sacral Segments

1 Coccygeal SegmentSpinal Cord - repeating similar units = cord segments,,

Dorsal root ganglionVentral rootSympathetic chain & ganglionDorsal rootSpinal nerveDorsal ramusOf spinal nerveVentralramusofspinal nerveWhite ramus communicans (WRC)Gray ramus communicans (GRC)DorsalVentral

Skin and muscles of the backSkin and musclesof the body walland limbsSENSORY (PPPTT = pain, pressure, proprioception,touch, temperature)MOTORSomatic innervation of the body wallDorsal ramusVentralramus

Sympathetics to Body WallSympatheticChain and Ganglia (paravertebral)ALL preganglionic sympathetics have cell bodies in the lateral horn of spinal gray matter from T1-L2; axons travel on spinal nerve ventral roots and enter the sympathetic chain via white rami communicantesFor spinal nerves above T1, preganglionic axons ascend in the chain to cervical chain gangliaFor spinal nerves below L2, preganglionic axons descend in the chain to lumbar and sacralchain ganglia

WRCAll preganglionic sympathetic cell bodies are located in the lateral horn of spinal segments T1L2: therefore, lateral horns are only located from T1-L2 (although there is something similar in segments S2-4)

All preganglionic sympathetic nerves enter the sympathetic chain via WRCs: therefore WRCs are only found in association with spinal nerves / chain ganglia T1L2 (above T1 and below L2 there are only GRCs)Segments T1-L2

GRCs onlyAbove T1 spinal cord segment and below L2 spinal cord segment, there are no lateral horns (no preganglionic sympathetic outflow above T1 or below L2)

There are no WRCs above T1 or below L2 for the same reason (no preganglionic sympathetic outflow to enter the chain above T1 or below L2)

GRCs are the on-ramps used by postganglionic sympathetics which are destined for the body wall and limbs GRCs ONLY lead to spinal nerves, the nerves of the body wall and limbs Segments above T1 and below L2Sympathetic fibers routes from the chain

WRCSegments T1-L2ALL preganglionic sympathetics enter the chain via WRCsFrom the chain, there are routes (everywhere):To the body wall and limbsTo thoracic organs (heart, airways, thoracic esophagus)To abdominal organsTo pelvic organsTo organs in the perineumTo organs/structures in the head

GRCs onlyWRCs and GRCsGRCs onlySympathetics to spinal nerves T1L2 preganglionics with cell bodies in lateral horn of spinal segments T1L2 enter the sympathetic chain via WRCs synapse in ganglia at the same spinal level postganglionics exit the chain via gray rami spinal nerves T1L2 dorsal and ventral rami of these spinal nerves innervate sweat glands , arrector pili muscles and vascular smooth muscle in the body wall at T1 L2 levelsSympathetics to Body Wall and LimbsSegments above T1 (cervical cord)Segments T1-L2Segments below L2

Segments above T1 (cervical cord)Upper Thoracic SegmentsSegments below L2GRCs onlyWRCs and GRCsGRCs onlySympathetics to Body Wall and LimbsSympathetics to spinal nerves above T1 preganglionics with cell bodies in lateral horn of upper thoracic segments enter the chain via WRCs ascend in chain synapse in cervical chain ganglia postganglionics exit chain via gray rami communicans spinal nerves C1-C8 dorsal and ventral rami of these spinal nerves supply neck / upper limbs above T1 level

GRCs onlyWRCs and GRCsGRCs onlySegments above T1 (cervical cord)Lower Thoracic, Segments, L1-L2Segments below L2Sympathetics to spinal nerves below L2 preganglionics with cell bodies in lateral horn of upper lumbar spinal segments enter the sympathetic chain via WRCs descend in chain synapse in lower lumbar and sacral chain ganglia postganglionics exit chain via gray rami spinal nerves L3-S5 dorsal and ventral rami of these spinal nerves innervate body wall / lower limbs below L2 levelSympathetics to Body Wall and Limbs

GRCs onlyWRCs and GRCsGRCs onlyWhat Targets Do Sympathetics Innervate In the Body Wall and Limbs?Segments above T1 (cervical cord)Segments T1-L2Segments below L2There are only 3 types of sympathetic targets In the body wall and limbs:Sweat glandsVascular smooth muscle (vessels in the skin, vessels in the connective tissues and vessels in skeletal muscles)Arrector pili muscles

Dorsal root is sensory only(and so far we have only seen somatic sensory nerves coming from the body wall and limbs via the dorsal and ventral rami of spinal nerves)

Ventral root is motor only, both somatic motor and preganglionic sympathetics (visceral motor)Somatic MotorPreganglionic

Spinal nerve and its branches (dorsal ramusand ventral ramus) are mixed nerves: somatic motor, somatic (and visceral) sensory and postganglionic sympathetics (visceral motor)Somatic MotorPostganglionicSensorySympathetic fibers routes from the chain to thoracic organs

Cardiopulmonary plexusEsophageal plexusA) Preganglionic sympathetics with cell bodies in lateral horn from T1-T4 enter chain via WRCs, synapse in chain ganglia, postganglionic axons exit the chain medially and travel to the cardiopulmonary plexusSegments T1-T4Sympathetic fibers routes from the chain to thoracic organs

Cardiopulmonary plexusB) Other preganglionic sympathetics with cell bodies in lateral horn from T1-T4 enter chain via WRCs, ascend in the chain and synapse in cervical chain ganglia, postganglionic axons exit the chain and travel to the cardio-pulmonary plexusSegments T1-T4Cervical cord / sympathetic chainSympathetic fibers routes from the chain to thoracic organs

Cardiopulmonary plexusDuring development, the heart descends through the cervical region to the thorax, picking up and carrying along some of its sympathetic innervation from cervical chain ganglia the ORIGIN of preganglionics is the same T1-4 lateral horns, etc.Segments T1-T4Cervical cord / sympathetic chain

Sympathetic fibers routes from the chain to abdominal organsPreganglionic axons with cell bodies in the lateral horn from T5L2 enter the chain, pass on through ganglia WITHOUT synapsing, then the (still) preganglionic fibers leave the medial aspect of the chain ganglia and travel to a sympathetic ganglion along the abdominal aorta (preaortic = prevertebral ganglia) where they synapse; postganglionic fibers then travel along arteries to the GI tractExamples Thoracic and lumbar splanchnic nervesGIPreaortic ganglia

Segments T5-L2

T5-9 = Greater thoracic splanchnic nerveT12 = Least thoracic splanchnic nerveT10-11 = Lesser thoracic splanchnic nerve

L1-2 = Lumbar splanchnic nerves

GIPreaortic gangliaThese sympathetics pass on beyond the chain still as preganglionics therefore they have to synapse in pre-aortic ganglia (variously named, e.g. celiac ganglion - the only other sympathetic ganglia beyond the chain)The post-ganglionic fibers follow arteries to the GI tractVisceral sensations are generally unconscious except for those that result in pain (which then becomes conscious).Visceral sensory fibers traveling with sympathetics and parasympathetics are called visceral afferentsGenerally, visceral afferents traveling with parasympathetics mediate unconscious sensations (such as atrial stretch or GI stretch) resulting in involuntary physiological responses, eg. enhanced peristalsis, decreased heart rate, etc.Generally, visceral afferents traveling with sympathetics mediate conscious sensations that are usually painful, eg. heart attack pain, gas pains, appendicitis pain, gall bladder pain, etc. and are caused by anoxia/ischemia, distension, inflammation, or spasmodic contraction of smooth muscles.VISCERAL SENSORY FIBERSVisceromotor (2 neuron chain)Visceral afferent (1 neuron)Target (organs in cavities)CNSPostganglionicPreganglionic

Visceral afferents for pain from heart (via cardiopulmonary nerves)PreganglionicWRCPostganglionicCardiopulmonary plexusVisceral afferentSkin, joints, tendons, etc.Somatic afferentSegments T1-T4

Visceral afferents from GI tract accompanying thoracic splanchnic nervesWRCGIPreaortic ganglionPreganglionicPostganglionicVisceral afferentSkin, joints, tendons, etc.Somatic afferentSegments T5-L2

Vagus n.(PSNS)Sympathetic chain ganglia (paravertebral ganglia)White and gray rami communicantesIntercostal nn.Sympathetic chainThoracic Sympathetic ChainSomatic ventral rami of thoracic spinal nervesEsophageal plexus (sympathetics, parasympathetics, and accompanying visceral afferents)

Cervical part of sympathetic chainGray rami communicantes from cervical chain/ganglia to cervical spinal nervesWhich of the following statements regarding gray rami communicans is most accurate with respect to principles of ANS anatomy? The GRCs are part of the route forpreganglionic SNS fibers destined for spinal nervespostganglionic SNS fibers destined for targets in the body wall/limbspostganglionic SNS fibers destined for the sympathetic chain(s)preganglionic PSNS fibers destined for visceral plexusespostganglionic PSNS fibers destined for targets in the body wall/limbs

Countdown20

WRCT1T4VagusThoracic chainCervical chainHeart (cardiopulmonary) autonomics

Greater thoracic splanchnic nerve (T5-9)Lesser thoracic splanchnic nerve (T10-11)Least thoracic splanchnic nerve (T12)

WRCChain ganglia -T5-9Pre-aortic ganglion (celiac)Greater thoracic splanchnic nerve

Sympathetic chain(lumbar part)Sympathetic chain(what part?)Superior hypogastricplexusIntermesenteric plexusPreaortic gangliaLumbar splanchnic nerve

Preganglionic parasympathetics with cell bodies in a small lateral-horn-like area of sacral (S2-4) segments of the spinal cord axons travel on the ventral (motor) roots to spinal nerves and then ventral rami of S2,3 and 4 then jump off the ventral rami as Pelvic Splanchnic Nerves to enter the inferior hypogastric (pelvic) plexus

Postganglionic parasympathetic nerves are located at or in the walls of target organs in the pelvis (such as the urinary bladder)Segments S2,3,4Pelvic splanchnic nerve(s)

Preaortic gangliaSacral sympathetic chain Superior hypogastric plexusS2,3,4 (Parasympathetic Pelvic Splanchnic Nerves)Pelvic (Inferior Hypogastric) PlexusSOME AXIOMS OF THE ANSAll sympathetic and parasympathetic fibers originate from the CNS. Thus, these nerves must be preganglionic (presynaptic).

All preganglionic sympathetics must enter the sympathetic chain from spinal nerves (or ventral rami of spinal nerves) via WRC.

All WRC are found only at spinal cord segments T1-L2. Thus, there are no WRCs above T1 spinal nerve or below L2 spinal nerve.

Some postganglionic sympathetics leave the sympathetic chain via GRCs to enter somatic (spinal) nerves. Thus, post-ganglionic sympathetic fibers traversing the GRC innervate body wall structures.SOME AXIOMS OF THE ANSALL somatic nerves contain postganglionic sympathetics but NO preganglionic sympathetics and NO parasympathetics. Thus, GRCs connect the sympathetic chain to ALL spinal nerves (allowing for passage of postganglionic sympathetic axons from the chain to the somatic nerves).

The term "splanchnic" only means visceral. Thus, the term splanchnic when used in thoracic splanchnics, lumbar splanchnics, sacral splanchnics and pelvic splanchnics only means that they are visceral nerves innervating appropriate visceral structures (and doesnt differentiate sympathetic from parasympathetic or pre-ganglionic from post-ganglionic).

Splanchnic nerves that leave the sympathetic chain ganglia may leave as pre- or post-ganglionic fibers, the preganglionics are destined for other ganglia (pre-aortic).

Pelvic splanchnics are preganglionic parasympathetics originating from spinal cord segments S2,3,4.